1. Field of the Invention
The present invention relates generally to the use of lasers, and more particularly to placement of their beams on a surface.
2. Background Information
A laser is a device that emits light (electromagnetic radiation) through stimulated emission. Typically, lasers emit light that is spatially coherent, meaning the light is emitted as a narrow, low-divergence beam, or is converted into one through the use of optical components such as lenses. Various uses of lasers have been developed since their inception, including, among other things, illuminating a surface, heating a surface (e.g., for printers), transmitting signals/data, and scribing (etching) or cutting a surface with a sufficiently powerful laser.
Often, precise placement of laser beams on an object or surface is required, such as during manufacturing processes, to create mechanical or electrical features by heating, scribing, cutting, etc. In some situations, the primary requirement is that the relative position of these features be separated by a specific distance, for instance, being as close as possible without risk of superposition or overlap of the features. For many of these situations, the exact placement of an initial feature is less important than the placement of additional features in relation to the initial feature.
As an example, in the field of solar panel manufacturing, thin films (e.g., monolithic) which constitute the active layers of photovoltaic solar panels may be scribed to divide coated areas into regions that are electrically isolated, by selective removal of coating material. Lasers are commonly used to make these scribes, where the laser beam scribe is focused to a “waist” (or “spot”) that is scanned along the desired path of the scribe, removing some or all of the layers which make up the photovoltaic coating. Typically, three or more scribes are required at different phases in the manufacturing process, for example, when isolating regions of a bottom conductive coating, creating an interconnect path or “via” to a top conductive coating, and isolating regions of the top conductive coating, e.g., repeated roughly every centimeter, resulting in a series electrical connections between individual solar panel cells. In this particular circumstance, it is essential to the operation of the solar panel that these scribes do not touch or cross one another. It is also desirable to place these scribes as close to one another as possible, because the photovoltaic material in the space between the scribes is non-functional in the generation of electricity, and therefore reduces the solar panel efficiency and electrical output.
There are a number of methods for placing and/or moving a laser spot along a surface. In general, these methods rely on repeatedly detecting (registering) the location of a workpiece/object, and placing the laser beam at a predetermined position on its surface based on the registrations. This approach, however, leads to an accumulation of positional tolerances which may be large relative to the desired spacing between the initial placement/pass and subsequent placements/passes of the scribe. To accommodate the positional tolerances, current laser beam placement techniques require that the nominal spacing between placements be increased in order to insure that subsequent placements do not overlap. For example, assume that a laser scribe placement has a positional tolerance of +/−2% on the surface of a workpiece. In order to account for the maximum degree of error to prevent overlap (e.g., 2% closer in each direction), the two scribes would need to be placed 4% apart. This planned 4% spacing could, in the extreme, result in scribes that are either touching (each being 2% closer to the other after the planned 4% offset), or scribes that are far apart (e.g., 8% apart, each being 2% further away from each other after the planned 4% offset).
As noted, for certain uses of the lasers, this varied placement between spots (e.g., scribes) on a surface may be undesirable or inefficient. In particular, it is often true that the primary requirement is that the laser beams be placed precisely relative to an entity on the surface, such as a previous scribe, while their absolute location on the surface is of secondary importance. There remains a need, therefore, for a technique that precisely places laser beams on a surface relative to other entities on the surface, particularly to entities created by previous placements of the beams.